Overview
Major new release with a complete overhaul of the LRU system. Potentially huge benefits in memory efficiency are possible if the new features are enabled. By default the code should behave similar to how it did in all previous versions, though locking is improved and the new code is still used in some ways.

Please read the feature notes carefully and try it out!

Real world examples have shown huge memory efficiency increases when using items of mixed TTL's (some short, some long). When all items have unlimited TTLs, hit ratios have still improved by several percent.

Fixes

spinlocks removed since they never seem to improve performance.

flush_all was not thread safe.

better handle items refcounted in tail by unlinking them from the LRU's

New Features
This release is a reworking of memcached's core LRU algorithm.

global cache_lock is gone, LRU's are now independently locked.

LRU's are now split between HOT, WARM, and COLD LRU's. New items enter the HOT LRU.

LRU updates only happen as items reach the bottom of an LRU. If active in HOT, stay in HOT, if active in WARM, stay in WARM. If active in COLD, move to WARM.

HOT/WARM each capped at 32% of memory available for that slab class. COLD is uncapped.

Items flow from HOT/WARM into COLD.

A background thread exists which shuffles items between/within the LRU's as capacities are reached.

The primary goal is to better protect active items from "scanning". items which are never hit again will flow from HOT, through COLD, and out the bottom. Items occasionally active (reaching COLD, but being hit before eviction), move to WARM. There they can stay relatively protected.

A secondary goal is to improve latency. The LRU locks are no longer used on item reads, only during sets and from the background thread. Also the background thread is likely to find expired items and release them back to the slab class asynchronously, which speeds up new allocations. Further work on the thread should improve this.

Overview
Bugfix release to replace 1.4.23. If you tried 1.4.23, please try this version instead. I apologize for any frustrations from the .23 release. If you see further issues, please report them quickly and we will look into them.